It is desirable in injection molding to urge the molten thermoplastic resin material into contact with the mold surfaces by exerting pressure on the resin. This aids the external surface of the plastic material in assuming the precise shape dictated by the mold surface. The pressure also assists the filling of the mold space with molten resin even if the space is elongated or narrow and is normally difficult to fill.
In gas assisted injection molding, the articles are produced by injecting molten resin into the mold cavity and injecting a quantity of pressurized gas into the resin to fill out the mold cavity and form a hollow portion in the resin. The gas is preferably an inert gas such, for example, as nitrogen. A pressure is maintained on the gas in the hollow gas space within the resin until the resin has sufficiently set, whereafter the pressurized gas is released from the molded part hollow space and the molded part is removed from the mold cavity.
This gas assisted procedure is advantageous since the molded part produced utilizes somewhat less plastic material and is lighter than if the part were solid plastic. More importantly, the plastic in the gas assisted procedure will not have a tendency to shrink away from the mold walls during cooling since the internal gas pressure will keep it pressed against the walls, thereby minimizing or eliminating surface blemishes such as sink marks. Further, the gas assisted procedure eliminates the need to utilize the screw ram of the injection molding machine to pack out the mold during the molding cycle, thereby minimizing or eliminating molded in stresses in the molded part.
Whereas the gas assisted injection molding process offers many advantages, some of which are enumerated above, as compared to injection molding without gas assistance, the known gas assistance processes incorporate several disadvantages. Specifically, prior art gas assisted processes often involve the use of custom made and expensive apparatus and equipment. Further, the prior art gas assisted processes and apparatus often require the use of pressurized oil to perform one or more steps in the process, with the result that system failure may occur when oil mixes with the nitrogen to contaminate the entire system and necessitate expensive repair to the system and attendant production losses, and with the further result that the times required to perform the oil actuated steps in the process are relatively long. Further, the prior art gas assisted processes and apparatus tend to be single purpose in nature and do not address different molding requirements, resin materials, or applications. Further, the prior art gas assisted processes and apparatus do not provide the precise control of the pressure, time and rate parameters required in the process with the result that the parts produced are often unsatisfactory.